[PATCH] tiny-printf: Add support for upper case hex values

[Marek Vasut]

On 3/20/25 3:00 PM, Quentin Schulz wrote:
> Hi Marek,
> 
> On 3/20/25 12:49 PM, Marek Vasut wrote:
>> On 3/20/25 11:23 AM, Christoph Niedermaier wrote:
>>> If tiny printf is used with 0x%08X (upper case X) the output is
>>> always 0x00000000. It could be confusing if upper case instead
>>> of lower case is used intentionally or accidentally because the
>>> actual value is not output. To avoid this confusion, tiny printf
>>> is extended to support also the formatting with %X.
>>>
>>> Signed-off-by: Christoph Niedermaier <cniedermaier at dh-electronics.com>
>> TINY_PRINTF is meant to be tiny, i.e. not consume a lot of space, at 
>> the expense of functionality. This is meant to be used in size 
>> constrained environments, like the SPL. If you need full vsprintf() 
>> formatting support, disable TINY_PRINTF in your config and use the 
>> regular vsprintf() implementation.
> 
> The issue is that disabling TINY_PRINTF may not be possible (size 
> constraints) and some code is compiled for different stages and people 
> typically don't check whether the format used in printf is valid with 
> tiny_printf. I've had this issue already in the past, I vaguely recall 
> "complaining" about it on IRC.
> 
> Maybe there's something we can do to verify that the code is working how 
> we expect it to work, regardless of tiny_printf/full printf selection? 
> checkpatch or a compile-time check for the formats maybe?
> 
> But yeah, essentially the whole thing is... if we continue like this, 
> we'll just end up getting closer and closer to the full printf which is 
> not something we want :)
Shall we maybe patch tiny printf to print '?' on unsupported formatting 
characters, or outright complain that users should fix their code ?

For the %x/%X thing, we could technically fall back from %X to %x , 
which would do the printing with minimum footprint increase, albeit 
slightly malformed:

diff --git a/lib/tiny-printf.c b/lib/tiny-printf.c
index 0503c17341f..48db7b1f78f 100644
--- a/lib/tiny-printf.c
+++ b/lib/tiny-printf.c
@@ -284,6 +284,7 @@ static int _vprintf(struct printf_info *info, const 
char *fmt, va_list va)
                                 islong = true;
                                 /* no break */
                         case 'x':
+                       case 'X':
                                 if (islong) {
                                         num = va_arg(va, unsigned long);
                                         div = 1UL << (sizeof(long) * 8 
- 4);